Indirectly heated cathode



July 4, 1939 F. D. GOODCHILD INDIRECTLY HEATED CATHODE Filed OCt. 19,1935 Fig. 1.

2 Sheets-Sheet 1 i'mi July 4, 1939. F. D. GOODCHILD INDIRECTLY HEATEDCATHODE Filed Oct. 19, 1935 2 Sheets-Sheet 2 Fig. 4..

Fig. 3.

Patented July 4, 1939 INDIRECTLY HEATED CATHODE Frank Douglas Goodchild,London, England, as-

signor to Iinternationa 1 Claim.

This invention relates to indirectly heated cathodes for thermionicvalves.

It is known to form a heater for an indirectly heated cathode in athermionic valve by coating a straight filament with insulating materialand then bending the filament into the shape required for insertion intothe cathode, as, for example, by bending the filament into a number ofloops which are inserted in a loose bundle into a metal tube of circularcross section which forms the cathode proper.

It is the object of this invention to form a heater which can be readilymaintained in any specified arrangement and in which a given length ofwire can be accommodated within less space than heretofore.

According to this invention a heater for an indirectly heated cathode ofa thermionic valve is formed by separately covering a plurality of wireswith insulating material, then placing the wires parallel and coatingthem with a second coating of insulating material and finally formingthe multiple wire into the shape required for insertion into thecathode.

This construction enables a given length of heater wire to be insertedinto a smaller space, and also enables the heater wire to be maintainedin a specified arrangement after folding into shape so that it canreadily be fitted to any particular form of cathode.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawings in which;

Figure 1 shows a heater for an indirectly heated cathode formedaccording to the invention from two wires, whilst Figs. 2 to 5 showvarious forms of cathode using a heater according to the invention.

In carrying out the invention tungsten wire is coated with'an insulatingcoating of, for example, aluminium oxide by any usual method such asspraying with or dipping it into a sludge of this material, which isafterwards dried to leave a coating of solid aluminium oxide. Thecoating is preferably not so thick as that applied when a single lengthof wire is to be folded to form the heater.

Two wires thus coated are then placed parallel to one another and againfed through the coating machine so that a second coating is formed overthe two wires together, the finished product being of the same form asthe well known twin bore lead covered cable. As an example, tungstenwire of .0018" (.046 mm.) is coated till its diameter is approximately.008.010 (.2-25 mm.). Two wires of this size are placed together andlightly coated until the maximum dimensions are .022" (.056 mm.) broadand .011"

i etandard Electric Corporation, New York, N. Y.

Application October 19, 1935, Serial No. 45,701 In Great Eritain March6, 1935 thick. The twin wire is then folded into the form required toinsert it into the oathode proper.

Figure 2 shows the manner in which the twin wire is folded into a bundleand disposed within a cylindrical cathode, whilst Fig. 1 shows onemanner in which the twin wire may be folded into a flat zig-zag form forinsertion into a cathode of flattened form. At the parts a where thetwin wire is bent the insulating coating breaks away, in the same manneras it does at the bends of a single wire, but with the double wire it isclear that there will be less danger of short circuits between theexposed tips of the loops.

Figure 3 shows the flat zig-zag of Figure l inserted into a cathode offiat rectangular shape.

With any form of cathode the two wires are cut at one end and therejoined as shown at b, Fig. 3. The other ends c, d of the two wires areconnected to suitable leads for the heating current which traverses onewire of the zig-zag and returns along the other wire.

A heater of cathode for insertion into a flattened form may be formedinto a series of flat wire above referred to. Figure 5 shows a fiattenedrectangular cathode into which has been inserted a zig-zag winding of atriple core wire. In this case the heating current enters by one outsidewire 0 at one end of the zig-zag and leaves by the other outside wire dat the other end of the zig-zag, the wire 0 and d being joined in themiddle wire at f and g respectively as shown.

What is claimed is:

An indirectly heated cathode for a thermionic Valve comprising anopen-ended casing of rectangular cross section having a greater lengththan breadth, an insulated structure of width approximately the internalbreadth of said cathode mounted in the interior thereof, said structurebeing lated cores composed of three separately insucovered as a unit toform a triple wire and folded into fiat zigzag form, each fold thereofconsisting of two separate parts, each part comprising a singleinsulated body having a triple wire core, and means interconnecting theend of said triple wire core in series, whereby a single seriesconductor arrangement is provided.

FRANK DOUGLAS GOODCI-IILD.

